1. Field of the Invention
The present invention relates to a method of forming thin metal films, and more particularly to a method of forming thin metal films by collimate sputtering which is utilized in the process of fabricating semiconductor devices.
2. Description of the Related Art
Recently, the designs of semiconductor devices are becoming increasingly dense and three-dimensional. Accordingly, connection pores (contact holes) for establishing connection with the conductor regions formed on semiconductor substrates have greatly increasing aspect ratios (depth-to-aperture ratios of connection pores). As a result, in cases where the metal films are mainly formed of aluminum by conventional sputtering, the metal films along the sidewalls of connection pores discontinue halfway, thus failing to establish satisfactory electric connection.
Blanket WCVD is widely used as a technique which solves the problem, and which also infills the connection pores for planarization. This process requires provision of an underlying film to ensure a lower resistance of the contact with the conductor region, adhesion of a tungsten film, and a barrier against the WF.sub.6 source gas. A tungsten film formed by sputtering is sometimes used as the underlying film, although this film is usually a laminate of a TiN film and a Ti film which does not need any special pretreatment for the film formation and has a small film stress. The Ti film is used as the lower layer which reduces the resistance of the contact with the conductor region formed on a semiconductor substrate, and the TiN film formed as the upper layer provides the barrier and adhesion properties.
The recent increase in aspect ratios of connection pores has become a great hindrance to the formation of the laminate of a TiN film and a Ti film by sputtering. As illustrated in FIG. 2, the coverage of the bottom of a connection pore (the bottom coverage, or the thickness of the film on the bottom of a connection pore/the thickness of the film on an insulating film) with a titanium film formed by the conventional sputtering is low, and steeply falls as the aspect ratio increases.
In addition, it is difficult to increase the thickness of a titanium film on the insulating film to 100 nm or greater. This is because increase in the thickness of the titanium film on the insulating film results in decrease in the aperture of the connection pore due to the titanium film, thus creating a bottleneck to infilling with tungsten. Furthermore, since titanium has a lower etching selection ratio than resists, increase in thickness of the titanium film makes it difficult to perform etching and formation of wiring patterns.
Since there is a 100-nm limit to the thickness of the titanium film on the insulating film, a bottom coverage of 10% or more is needed to ensure 10-nm film thickness required to satisfactorily lower and stabilize the resistance of the contact at the bottom of the connection pore. Nevertheless, as illustrated in FIG. 2, the bottom coverage by a titanium film using conventional sputtering is 10% or less when the aspect ratio is lower than 2. Therefore, it is difficult to ensure excellent contact properties when the connection pore has an aspect ratio of 2 or higher.
As means of solving the problem, collimate sputtering which uses a collimator, a porous plate, placed between the target and the semiconductor substrate has been suggested (see Japanese Examined Patent Application HEI 6-60391, and Japanese Unexamined Patent Application Disclosure HEI 1-116070, for example). In order to form a film, the collimator captures sputtered particles which are scattered far off the direction of the normal to the target (the direction of the normal to the collimator plate .+-.0.79 rads when the length-to-diameter ratio of each pore of the collimator plate is 1:1). This increases the ratio of the sputtered particles which reach the bottom of the connection pore with a high aspect ratio, and thus improves the bottom coverage.
FIG. 2 shows the aspect ratio-dependency of the bottom coverage when collimate sputtering is used. The aspect ratio of the collimator (the length-to-aperture ratio of each pore of the collimator) used in the case shown in the drawing is 1. The target used here is a titanium target with a high orientation rate for the (002) plane, which will be described in detail later.
The bottom coverage is approximately 15% when the aspect ratio of the contact hole is 2. Accordingly, even in the case of connection pores with high aspect ratios, it is possible to form titanium films having satisfactory thicknesses of 10 nm at the bottoms of the connection pores without increasing the thicknesses of the titanium films on the insulating films to 100 nm or greater.
In addition, use of a collimator with a higher aspect ratio allows only the sputtered particles near the direction of the normal to the semiconductor substrate to strike the semiconductor substrate, and this results in a higher bottom coverage which allows for contact holes with higher aspect ratios.
A problem of the prior art techniques is that the conventional collimate sputtering cannot achieve both an increase in the productivity, and the necessary degree of bottom coverage to provide contact holes with aspect ratios of 2.5 or higher, with excellent electrical characteristics.
The reasons are as follows.
When the aspect ratio of the collimator is 1, the bottom coverage for a connection pore with an aspect ratio of 2.5 is 10% or less, as illustrated in FIG. 2. As described in the section "Description of the Prior Art", it is impossible to ensure a satisfactory titanium film thickness at the bottom of the connection pore in the above case.
Collimate sputtering allows the bottom coverage to be easily increased by increasing the aspect ratio of the collimator, since only the particles are allowed to pass through. However, this also results in more of the sputtered particles being captured by the collimator. For example, when the aspect ratio of the collimator is increased from 1 to 1.5, the ratio of the sputtered particles which reach the substrate decreases to 1/2. As a result, the film forming rate per unit power decreases to 1/2, and the frequency of replacement of the target and the collimator is doubled.
The conventional collimate sputtering fails to provide satisfactorily high bottom coverages as compared with the aspect ratio of the collimator for the following reasons.
The titanium targets used for the collimate sputtering of the prior art are the same as the targets used for the conventional sputtering (sputtering without use of a collimator). In other words, the targets are not particularly designed so as to match collimate sputtering. The titanium targets have a crystal orientation wherein the sputtering surfaces are greatly orientated in the (002) plane.
The collimator passage rate of the sputtered particles depends on the angle of the sputtered particles incident on the collimator plate. Assuming that the direction of the normal to the collimator plate is the direction of 0 rads, the passage rate of the sputtered particles gradually decreases until the angle reaches a limit passage angle (0.79 rads when the collimator has an aspect ratio of 1). When the collimator has an aspect ratio of 1, the passage rate for angles around 0.25 rads is approximately 50% that for the direction of 0 rads. When the aspect ratio of the collimator is 1, the direction which allows the sputtered particles to pass through ranges from 0 to 0.25 rads. Since the target plate and the collimator plate are placed in parallel in a sputtering chamber, mainly particles emitted from the target with angles within the above range pass through the collimator.
As mentioned above, the titanium target used in the conventional collimate sputtering has a sputtering surface greatly orientated in the (002) plane. The emission angle distribution of the sputtered particles when the sputtering surface is in the (002) plane is as illustrated in FIG. 1. The drawing illustrates the results of a simulation performed by the inventors. The direction of the normal to the target is defined to be the direction of 0 rads also in the drawing. The distribution of angles of emission from the (002) plane does not exhibit a particularly high emission probability for the direction of 0 rads to 0.25 rads, in which the sputtered particles have a high passage rate for the collimator with an aspect ratio of 1:1. For this reason, a satisfactorily high bottom coverage is not achieved.